Systems and approaches for semiconductor metrology and surface analysis using secondary ion mass spectrometry

ABSTRACT

Systems and approaches for semiconductor metrology and surface analysis using Secondary Ion Mass Spectrometry (SIMS) are disclosed. In an example, a secondary ion mass spectrometry (SIMS) system includes a sample stage. A primary ion beam is directed to the sample stage. An extraction lens is directed at the sample stage. The extraction lens is configured to provide a low extraction field for secondary ions emitted from a sample on the sample stage. A magnetic sector spectrograph is coupled to the extraction lens along an optical path of the SIMS system. The magnetic sector spectrograph includes an electrostatic analyzer (ESA) coupled to a magnetic sector analyzer (MSA).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/114,521, filed on Feb. 10, 2015, the entire contents of which arehereby incorporated by reference herein. This application also claimsthe benefit of U.S. Provisional Application No. 62/114,519, filed onFeb. 10, 2015, the entire contents of which are hereby incorporated byreference herein. This application also claims the benefit of U.S.Provisional Application No. 62/114,524, filed on Feb. 10, 2015, theentire contents of which are hereby incorporated by reference herein.

BACKGROUND 1) Field

Embodiments of the invention are in the field of semiconductor metrologyand, in particular, systems and approaches for semiconductor metrologyand surface analysis using Secondary Ion Mass Spectrometry (SIMS).

2) Description of Related Art

Secondary ion mass spectrometry (SIMS) is a technique used to analyzethe composition of solid surfaces and thin films by sputtering thesurface of the specimen with a focused primary ion beam and collectingand analyzing ejected secondary ions. The mass/charge ratios of thesesecondary ions are measured with a mass spectrometer to determine theelemental, isotopic, or molecular composition of the surface to a depthof 1 to 2 nanometers. Due to the large variation in ionizationprobabilities among different materials, SIMS is generally considered tobe a qualitative technique, although quantitation is possible with theuse of standards. SIMS is the most sensitive surface analysis technique,with elemental detection limits ranging from parts per million to partsper billion.

Generally, SIMS requires a high vacuum with pressures below 10⁻⁴ Pa.High vacuum is needed to ensure that secondary ions do not collide withbackground gases on their way to the detector (i.e., the mean free pathof gas molecules within the detector must be large compared to the sizeof the instrument), and it also prevents surface contamination thatotherwise may occur by adsorption of background gas particles duringmeasurement.

Three basic types of ion guns are employed in SIMS analysis. In one,ions of gaseous elements are usually generated with duoplasmatrons or byelectron ionization, for instance noble gases (⁴⁰Ar⁺, Xe⁺), oxygen(¹⁶O⁻, ¹⁶O²⁺, ¹⁶O²⁻), or even ionized molecules such as SF₅ ⁺ (generatedfrom SF₆) or C₆₀ ⁺ (fullerene). This type of ion gun is easy to operateand generates roughly focused but high current ion beams. A secondsource type, the surface ionization source, generates ¹³³Cs⁺ primaryions. Cesium atoms vaporize through a porous tungsten plug and areionized during evaporation. Depending on the gun design, fine focus orhigh current can be obtained. A third source type, the liquid metal iongun (LMIG), operates with metals or metallic alloys, which are liquid atroom temperature or slightly above. The liquid metal covers a tungstentip and emits ions under influence of an intense electric field. While agallium source is able to operate with elemental gallium, recentlydeveloped sources for gold, indium and bismuth use alloys which lowertheir melting points. The LMIG provides a tightly focused ion beam (lessthan 50 nanometers) with moderate intensity and is additionally able togenerate short pulsed ion beams. It is therefore commonly used in staticSIMS devices.

The choice of the ion species and ion gun respectively depends on therequired current (pulsed or continuous), the required beam dimensions ofthe primary ion beam and on the sample which is to be analyzed. Oxygenprimary ions are often used to investigate electropositive elements dueto an increase of the generation probability of positive secondary ions,while cesium primary ions often are used when electronegative elementsare being investigated. For short pulsed ion beams in static SIMS, LMIGsare most often deployed for analysis. LMIGs can be combined with eitheran oxygen gun or a cesium gun during elemental depth profiling, or witha C₆₀ ⁺ or gas cluster ion source during molecular depth profiling.

Depending on the SIMS type, there are three basic analyzers available:sector, quadrupole, and time-of-flight. A sector field mass spectrometeruses a combination of an electrostatic analyzer and a magnetic analyzerto separate the secondary ions by their mass to charge ratio. Aquadrupole mass analyzer separates the masses by resonant electricfields, which allow only the selected masses to pass through. The timeof flight mass analyzer separates the ions in a field-free drift pathaccording to their velocity. Since all ions possess the same kineticenergy, the velocity and therefore time of flight varies according tomass. The time of flight mass analyzer requires pulsed secondary iongeneration using either a pulsed primary ion gun or a pulsed secondaryion extraction. The time of flight mass analyzer is the only analyzertype able to detect all generated secondary ions simultaneously, and isthe standard analyzer for static SIMS instruments.

In the field of surface analysis, it is usual to distinguish static SIMSand dynamic SIMS. Static SIMS is the process involved in surface atomicmonolayer analysis, or surface molecular analysis, usually with a pulsedion beam and a time of flight mass spectrometer. Meanwhile, dynamic SIMSis the process involved in bulk analysis, closely related to thesputtering process. Dynamic SIMS employs a DC primary ion beam and amagnetic sector or quadrupole mass spectrometer.

SIMS is a very powerful technique. However, advances are needed in thearea of SIMS measurement equipment, systems, and methodologies.

SUMMARY

Embodiments of the present invention includes systems and approaches forsemiconductor metrology and surface analysis using Secondary Ion MassSpectrometry (SIMS).

In an embodiment, a secondary ion mass spectrometry (SIMS) systemincludes a sample stage. A primary ion beam is directed to the samplestage. An extraction lens is directed at the sample stage. Theextraction lens is configured to provide a low extraction field forsecondary ions emitted from a sample on the sample stage. A magneticsector spectrograph is coupled to the extraction lens along an opticalpath of the SIMS system. The magnetic sector spectrograph includes anelectrostatic analyzer (ESA) coupled to a magnetic sector analyzer(MSA).

In an embodiment, a method of measurement and control of the surfacepotential of a sample involves measuring kinetic energy of chargedparticles emitted from a surface of a sample. The method also involvesdetermining a shift in kinetic energy of the charged particles. Thesurface potential of the surface of the sample is changed in responsethe shift in kinetic energy of the charged particles.

In an embodiment, a method of determining wafer backside contactresistance involves measuring a gap distance value of a surface of awafer based on a comparison of a main capacitive sensor electrode drivenwith a first drive signal and a compensating capacitive sensor electrodedriven with a second drive signal that is amplitude or phase shifted ascompared to the first drive signal. A value of the second drive signalis measured. The value of the second drive signal is calibrated to areference impendence standard to determine an impedance value of thewafer to ground. A contact resistance value is determined for thesurface of the wafer based on the gap distance value and the impedancevalue of the wafer to ground.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic layout of a SIMS measurement system, inaccordance with an embodiment of the present invention.

FIG. 2 illustrates schematically charge compensation considerations fora SIMS system, in accordance with an embodiment of the presentinvention.

FIG. 3 illustrates a stage area that includes a Faraday cup and an areafor storing calibration standards, in accordance with an embodiment ofthe present invention.

FIGS. 4A and 4B include a schematic representation for the use of asystem of electrostatic analyzer and an energy slit equipped withcurrent sensor to monitor and control a charge compensation system, inaccordance with an embodiment of the present invention.

FIG. 5 illustrates a secondary ion beam path through an upperspectrometer and turning electrostatic analyzer (ESA) section, inaccordance with an embodiment of the present invention.

FIG. 6 illustrates a secondary ion beam path for positive ions throughan upper spectrometer and turning electrostatic analyzer (ESA) section,in accordance with an embodiment of the present invention.

FIG. 7 illustrates an energy distribution scan toward high energy, usingthe low and high energy current sensors following turning ESA 116 asfeedback which changes the mean pass energy of the electrostaticanalyzer, in accordance with an embodiment of the present invention.

FIGS. 8A, 8B, 8C, and 8D include a schematic illustration for measuringthe secondary ion energy distribution using the electrostatic turningESA 116, as well as the high- and low-energy slit sensors described inassociation with FIG. 7, in accordance with an embodiment of the presentinvention.

FIG. 9 illustrates an energy distribution scan involving turning ESAscan on ESA 116 and ESA 118 at a same voltage, in accordance with anembodiment of the present invention.

FIG. 10 is a schematic illustrating a positive potential on the sampledue to surface charging, in accordance with an embodiment of the presentinvention.

FIG. 11 is a schematic illustrating electron beam based chargeneutralization and control, in accordance with an embodiment of thepresent invention.

FIG. 12 is a schematic illustrating photo conductivity of the sampleusing a variable intensity photon source, in accordance with anembodiment of the present invention.

FIG. 13A is a schematic illustrating charge neutralization by use of anelectron emitting material away from the sample, in accordance with anembodiment of the present invention.

FIG. 13B is a schematic illustrating effective charge neutralization ifthe electron flux exceeds the primary ion induced positive surfacecharge, in accordance with an embodiment of the present invention.

FIG. 14 illustrates schematically some of the parameters involved inusing a compensating electrode to determine gap distance.

FIG. 15 illustrates a block diagram of an exemplary computer system, inaccordance with an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Systems and approaches for semiconductor metrology and surface analysisusing Secondary Ion Mass Spectrometry (SIMS) are described. In thefollowing description, numerous specific details are set forth, such asSIMS analysis techniques and system configurations, in order to providea thorough understanding of embodiments of the present invention It willbe apparent to one skilled in the art that embodiments of the presentinvention may be practiced without these specific details. In otherinstances, well-known features such as entire semiconductor devicestacks are not described in detail in order to not unnecessarily obscureembodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the Figures areillustrative representations and are not necessarily drawn to scale.

In a first aspect, embodiments of the present invention are directed tosystems and methods for characterization and process control ofstructures on semiconductor wafers during manufacturing using SecondaryIon Mass Spectrometry (SIMS).

To provide context, state of the art semiconductor manufacturing entailsmany hundreds of process operations. Each process operations is subjectto processing variations which can lead to variation and degradation ofthe performance of the finished device. As such, many measurements andinspections are performed during the manufacturing process to bothensure that a manufacturing operation has been executed correctly andwithin specification and also to provide feedforward and feedback tocontrol the manufacturing process. Process control requires several keyattributes that enable its effectiveness.

First, the cost of a measurement or inspection must be consistent withthe economical manufacturing of devices. Practically, this means thatthe cost of a tool, the cost of consumables, the cost of ownership(including maintenance, calibration and other costs), its usefullifetime, the time required to take a measurement and the labor costsassociated with its effective use are consistent with the manufacturingenvironment. Perhaps most importantly, it is critical that thecharacterization be nondestructive, as the cost of a semiconductor waferduring processing can be very high, and techniques which requiredestructive testing are not necessarily viable.

Second, the inspection and measurement must measure a sample which isdirectly relevant to the process control requirements. While techniquesthat can only measure “monitor” wafers, i.e., those wafers that haveonly undergone one or a few process operations, can be useful for toolqualification or coarse quality control, effective process controlrequires that the measured structures are as similar to actual devicestructures as possible and that measurement sampling can be as frequentas possible compared to the rate of process variation and drift.

Third, and perhaps most importantly, the inspection or measurementoperation must have the desired information content, i.e., the requiredrelevancy, resolution, accuracy and precision, required for processcontrol.

To provide further context, Secondary Ion Mass Spectrometry (SIMS) is awell-known materials characterization technique which is capable ofdetermining the depth profile of elemental composition with greataccuracy and precision. Because of this unique capability, SIMS iswidely used in materials science and analytical laboratory settings.While SIMS is currently used in an “offline” mode in semiconductormanufacturing, its use in “inline” process control is currently notviable due to its inability to address the practical issues outlinedabove. What is needed, therefore, is a Secondary Ion Mass Spectrometrysystem which provides the well known capabilities of SIMS whileaddressing the gaps which prevent its use in inline process control.

In order to satisfy the requirements of an in-line semiconductormanufacturing environment, several gaps in current SIMS designs must beaddressed. Such gaps include particular challenges associated withmeasurement on typical product wafers, the time it takes to make themeasurements, the availability/uptime of the tool (e.g., including timefor calibration and verification), and the costs associated with themaintenance of the tool. Design specifics addressing one or more of theabove issues are described below, in accordance with one or moreembodiments of the present invention.

With respect to measurement on product wafers, performing a SIMSmeasurement on a product wafer entails three challenges that constrainthe design. First, measurements on product wafers must typically beconfined to a small designated area, such as a 50 μm by 50 μm pad.Second, at the same time, the manufacturer is typically interested inchanges of composition over very small changes in depth. Third, therewill be an unpredictable grounding path to the measurement site. Incases where the grounding is poor or non-existent the wafer will chargeduring the SIMS measurement, which leads to several problems including adeflection of the primary beam away from the intended site, a decreasein detected secondary ion signal, and a reduction in the ability tocorrectly identify and resolve secondary ion masses.

Addressing one or more of the above issues concerning measurementsperformed on product wafer, in accordance with an embodiment of thepresent invention, a SIMS approach described herein involves use of alow extraction Voltage at a secondary extraction lens of a SIMSmeasurement system. In particular, only a very weak electric field isused between the collection lens and the sample under measurement. Suchan approach is contrary to a typical SIMS design which uses a strongelectric field between the extraction lens and the sample to attract thesecondary ions towards the lens and boost the number of collected ions.In one embodiment, the extraction field threshold is at mostapproximately 10 Volts/mm (as an absolute value).

FIG. 1 illustrates a schematic layout of a SIMS measurement system 100,in accordance with an embodiment of the present invention.

Referring to FIG. 1, the SIMS measurement system 100 includes an XYstage for holding a sample for analysis. Note that the stage need not beXY but could be any type such as XYZ, XY-Theta, R-Theta, etc. The XYstage may include a Faraday cup and fiducials, as described in greaterdetail below. A primary ion beam 104, which is generated by an ionsource, refined by mass filters, and focused with a series of ionoptics, is directed to the XY stage 102, e.g., at an incident angle of40 degrees. A charge compensation electron gun 106 may also be directedat the XY stage, as is depicted in FIG. 1, An extraction lens 108 ispositioned over the XY stage 102. In one embodiment, the extraction lens108 has a portion near ground potential to maintain a small extractionfield between the lens and the sample, typically of the order 10V/mm orless. A secondary ion beam 110 is collected from a sample on the XYstage into the extraction lens 108 and into collection/detection pathsof SIMS system 100. It is to be appreciated that, although not depicted,the SIMS measurement system 100 of FIG. 1 may be included in anenclosure or chamber with vacuum environment.

Referring again to FIG. 1, an array of elements 112 may be includedalong the path 110 following the extraction lens 108. The array ofelements 112 may include secondary-ion optics, deflectors and/oradditional compensation mechanisms, as shown. The secondary ion beam 110is then directed through a slit 114 and then into a first electrostaticanalyzer (ESA) 116 and a second ESA 118. A slit 120 is placed betweenthe first 116 and second 118 ESAs. The slit may be placed at thelocation where the beam 110 is spread by energy. In the SIMS system 100,the two ESAs 116 and 118 direct the beam toward the floor of the SIMSsystem 100. The two ESAs 116 and 118 also create a point at which thebeam is spread according to energy, which can serve as a signal (e.g.,along signal line 121) for the charge compensation system 106.

Referring again to FIG. 1, the beam 110 is then directed through a slit120A and other features. For example wired slits and apertures 122 maybe placed at several points along the beam path 110 to both limit/shapethe beam and to monitor currents and beam positions. Many of these areon actuators to enable movement. Various other elements 124 such asoptics and deflectors are included along the path 110 to keep the beamaligned and properly shaped.

Referring again to FIG. 1, the beam 110 is then directed through a slit126 and into a third ESA 128, exit through slit 130 and throughadditional optics 132. The beam 110 is then directed into a magneticsector analyzer (MSA) 134. The MSA 134 includes multiple (e.g., 10)detectors 136 along a focal plane 138, enabling detection of up to 10species in parallel, as is described in greater detail below. It is tobe appreciated that the MSA 134 and the third. ESA 128 togetherconstitute a magnetic sector spectrograph.

In one embodiment, referring again to system 100, by using a lowextraction Voltage between the extraction lens 108 and the sample 102,as much as 80% of a signal otherwise obtainable by a typical SIMSmeasurement is sacrificed. However, in return, it is much easier toaccurately place the primary beam. For example, in systems where theextraction field is strong, the primary beam is deflected as much astens or even hundreds of microns as it passes through the field, whichmeans that the aim of the primary beam must be altered to compensate.This leads to a tricky control problem with typical SIMS measurements,making the precise rastering of the beam in a small 50 μm area verydifficult.

Such deflection of the primary beam by the extraction field is moresevere the lower the energy of the primary ions. Accordingly, undertypical or conventional SIMS operation, there is incentive to maintain ahigh primary energy in order to minimize the beam deflection. However,high-energy primary beams lead to two effects that degrade the depthresolution. First, a high-energy primary beam penetrates more deeplyinto the sample leading to ions being created over a range of depthsrather than at the surface. Second, the higher energies can drivesurface atoms of the sample deep into the substrate, essentiallysmearing the material over depth. In an embodiment, in contrast tohigh-energy primary beams used in typical SIMS measurements, fine depthresolution required by the semiconductor industry is benefited from useof a low-energy primary beam. Accordingly, in one embodiment, the abovedescribed advantage of using a low-extraction field design is evengreater when also designing for maximum depth resolution since otherwisesevere deflection of a low-energy primary beam is mitigated by thelow-extraction field design.

Additionally, in an embodiment, using a low extraction field makes iteasier to deal with sample charging in cases of poor grounding. Forexample, reducing the field above the sample enables the use of alow-energy electron flood to neutralize the charge. By contrast, in amore typical SIMS design using a strong extraction field, acharge-compensating beam must be run at high energy and preciselysteered in order to overcome the deflection by the field and to placethe beam accurately at the measurement site, which is a vastly morecomplicated process. FIG. 2 illustrates schematically chargecompensation considerations for a SIMS system. In accordance with anexemplary embodiment, details of a charge compensation system aredescribed in greater detail below.

Referring to FIG. 2, a portion of the system 100 of FIG. 1 isillustrated on the left-hand side, particularly, the section includingthe first and second ESAs 116 and 118 and the charge compensation system106. In a first scenario 200, if an analyzed wafer is at groundpotential then the ion beam will nominally be located half way betweenthe slits 120. The tails of the beam will provide small signals to eachof two current detectors 201. In a second scenario 202, if the wafercharges then the ion beam will shift toward one of the slits 120,dependent upon the sign of the charging. The current in the twodetectors 201 will change, which may be used to control the chargecompensation system 106.

With respect to throughput, the time needed to make a set ofmeasurements on a wafer is a critical factor in the economicalapplication of a measurement technique. In accordance with one or moreembodiment of the present invention several secondary ion species aremeasured in parallel for increased throughput.

To provide context for throughput considerations, common SIMS designscan be grouped into three categories: (1) time-of-flight (TOF), (2)quadrupole detection, and (3) magnetic sector detection. In TOF SIMS,the primary beam is pulsed and the sputtered secondary ion species aresorted according to the time it takes for them to reach a detector. TOFSIMS has the advantage that a wide range of secondary ion species arecollected in parallel, but the pulsed nature of the measurement reducesthe measurement speed. In the quadrupole approach, the primary beam isapplied continuously, but the detector can only measure one ion speciesat a time (although switching from one species to another can be donequickly). Finally, the third common approach, magnetic sector, separatesthe ion species according to mass/charge using a combination of electricand magnetic fields. As with the quadrupole system, the primary beam ison continuously. However, in typical magnetic sector systems thespectrometer uses a single detector, which again limits the detection toa single species at a time.

None of the above conventional approaches provide the speed and paralleldetection possibly needed for the semiconductor manufacturingenvironment. By contrast, in accordance with one or more embodiments ofthe present invention, a magnetic sector spectrograph is used. Like thespectrometer, a magnetic sector spectrograph separates the various massspecies by sending the beam through a magnetic field. However, thespectrograph design focuses the different masses along a line, or “focalplane.” By placing multiple detectors at various positions along thefocal plane, different masses can be measured in parallel. In aparticular embodiment, a SIMS system design includes 10 differentdetectors, enabling measurement of a vast majority of species present inany single semiconductor measurement. It is to be appreciated that fewerthan or greater than 10 detectors may be used. It is also to beappreciated that although the entire available range of masses may notbe collected in parallel in such an arrangement, as is the case for TOFSIMS, the multi-detector approach is not a detriment since the number ofspecies typically used in any given semiconductor process is limited andthe manufacturer usually has a good notion of which species are likelyto be present. It should also be noted that the plurality of detectorscan be mounted onto translation stages so that their locations along thefocal plane, and hence the collected species, can be tailored accordingto the needs of a specific measurement.

With respect to availability and serviceability, due to thetime-critical nature of process control measurements, it is essentialthat the tool be available for use at the time when a wafer becomesavailable for measurement. For this reason, it is important that themean time between failures of the tool is long and the mean time torepair is short. It is further required that the measurement tool is incalibration at all times. Accordingly, the SIMS tool must be capable ofchecking and updating its calibration automatically.

In accordance with one or more embodiments herein, several designaspects of a SIMS system are tailored to address such issues concerningavailability and serviceability. In one embodiment, to improve theoverall stability of the hardware, a monolithic system structure isimplemented. For example, every major sub-system, such as the extractionoptics, the lower primary column, and the mass spectrometer, may berigidly mounted to one another. There may be a tradeoff with respect toalignment flexibility, but the advantages include a resulting systemthat can hold on to an alignment more reliably. In addition, in oneembodiment, as depicted in FIG. 1, two turning electrostatic analyzers(ESAs) are added to a secondary path to divert the beam in a directionthat allows a several-hundred pound spectrometer magnet to be mountedbelow a main measurement chamber, near the floor. Such an arrangementimproves service access and vibration stability.

In accordance with one or more embodiments herein, to maintainmeasurement consistency, several internal checks and calibrations areadded to a SIMS system. Such internal checks and calibrations mayinclude one or more of (1) a Faraday cup, as depicted in FIG. 1, with afiducial overlay to verify the current, position, and focus of theprimary beam, and/or (2) the ability to read current at severalapertures and slits along the secondary path, as depicted in FIG. 1,including the ability to mechanically scan slits to verify the alignmentof the beam and monitor any aperture wear.

In an embodiment, a set of internal reference samples is included in aSIMS system. The internal samples can be automatically moved to themeasurement site to periodically check various crucial aspects of thebeam. These may include layered materials of predetermined thickness tomonitor the primary sputtering rates and raster uniformity, patternedboxes of known dimensions to check the raster size and position, andsimple chips of various composition to monitor the overall signalstrengths. Many of the tests can be performed at a multitude of primaryenergies and currents. As an example, FIG. 3 illustrates a stage areathat includes a Faraday cup and an area for storing calibrationstandards, in accordance with an embodiment of the present invention.

Referring to FIG. 3, a portion of the system 100 of FIG. 1 isillustrated on the left-hand side, particularly, the section includingthe XY stage 102 and the extraction lens 108. A wafer or sample 103 issituated on the XY stage 102, below the extraction lens 104. The XYstage 300 includes a Faraday cup 300, a top view of which is also shownin FIG. 3. A transfer robot 302 may also be included, along with a“parking lot” for one or more reference wafers such as layered films ofknown composition and/or film thickness.

In a second aspect, one or more embodiments involve measuring thesurface potential of a sample by observing the shift in kinetic energyof emitted charged particles or ions. One or more embodiments involvemeasuring and controlling the surface potential by using shifts ofkinetic energy as a feedback mechanism.

To provide context, secondary ions are generated by removing atoms froma surface, some of which are ionized. The secondary ions are emittedwith a range of velocities, or equivalently, a range of kineticenergies. As the ions travel along an optical path, their kinetic energyat an arbitrary point A will be the sum of the initial kinetic energy(KE), the surface potential of the sample, the local potential at A, andany KE contribution from electrodynamic elements (e.g., buncher,cyclotron, etc.). Determining the KE distribution and comparing it to adistribution from a reference sample with known surface potentialprovides a measure of the sample potential.

A primary reason to measure surface potential is charging of insulatingsamples caused by an unequal flow of positive and negative charges tothe sample. Secondary ions may be formed by impinging atoms, ions,electrons, or photons onto a surface. Ions and electrons add charge tothe surface, while any secondary charged particles emitted from thesurface removes charge. If the sample is electrically conducting, itwill dissipate any imbalance of charge transport. Insulating overlayers(e.g., which in some cases are laterally patterned) may charge, and theentire sample may charge if it is not electrically connected to anelectrode at a stable potential. Furthermore, the surface potential maybe measured as material is removed from the surface, possibly exposingvariations in sample conductivity or in the emission of chargedparticles.

In accordance with an embodiment of the present invention, the emittedsecondary ions are passed through an electrostatic analyzer so that ionsare dispersed in space according to their energies, The complete KEdistribution is measured by varying the electric field in the ESA, suchthat the dispersed ion beam travels across an ion detector 120. A changein KE can be measured while intercepting only the low and/or high tailsof the energy distribution by using one or two detectors positioned atthe tails. In one embodiment, the slope of a tail is measured byslightly modulating the ESA deflecting field. To preserve the positionof the main transmitted beam, the apparatus may have an arrangement oftwo ESAs with the ion detector(s) 120 there between. In one suchembodiment, the second ESA, or the second ESA in conjunction with amatching lens, has an equal and opposite dispersive strength to that ofthe first ESA.

Particular embodiments of the present invention are directed tomeasurement and control of the surface potential of a sample byobserving the shift in kinetic energy of emitted charged particles andby using shifts of kinetic energy as a feedback mechanism. It is to beappreciated that although exemplary embodiments are described inassociation with SIMS measurements, embodiments described herein may beapplicable to other measurement and metrology techniques and systems.

One or more embodiments are directed to a system of at least oneelectrostatic analyzer with an energy slit that is equipped with ioncurrent sensors. One or more embodiments are directed to a system and/orapproach for measuring energy distribution and energy shift of the totalsecondary charged particles transported from a sample surface prior to amass analyzer. One or more embodiments are directed to implementation ofan electrostatic analyzer and energy slit feedback system to monitorsurface charging. One or more embodiments are directed to utilization ofan active control system to adjust the surface potential. In anembodiment, the system of at least one electrostatic analyzer with anenergy slit that is equipped with ion current sensors, the system and/orapproach for measuring energy distribution and energy shift of the totalsecondary charged particles transported from a sample surface prior to amass analyzer, and the electrostatic analyzer and energy slit feedbacksystem implemented to monitor surface charging are used together as afeedback system for surface potential changes.

In an embodiment applicable to SIMS measurements in which the main ionbeam is sent through a mass spectrometer, it is often analyticallydesirable to reject secondary ions which originate near the edge of theeroded area, since these can confound a depth profile. The complete KEdistribution may be measured while the main beam is rejected. In oneembodiment, the tails of the distribution are measured while the mainbeam should travel through the mass spectrometer.

FIGS. 4A and 4B include a schematic representation for the use of asystem of electrostatic analyzer and an energy slit equipped withcurrent sensor to monitor and control a charge compensation system, inaccordance with an embodiment of the present invention.

Referring to FIG. 4A, a first portion of a flowchart 400 begins withoperation 402 to determine if charge compensation is needed. If nocharge compensation is needed, then charge compensation is OFF atoperation 404. This is confirmed with a charge compensation slit at aturning ESA at operation 406 and progresses to FIG. 4B at 408. If chargecompensation is needed, then charge compensation is ON at operation 410.This is monitored with a charge compensation slit at a turning ESA atoperation 412. At operation 414, a symptom is determined to see if nosignal on low energy charge compensation (CC) Jaw/Si intensity fadesaway. If yes, the determination progresses to FIG. 4B at 416. If no, thedetermination progresses to FIG. 4B at 418.

Referring to FIG. 4B, pathway 408 continues to operation 420 which is a“Go” for running the analysis. Pathway 416 continues to operation 422 todetermine if the signal continues to decrease. At operation 424,adjustments are made to the charge compensation. At operation 426,monitoring is performed with a charge compensation slit at a turningESA. At operation 428, overall energy distribution is measured.

Referring again to FIG. 4B, pathway 418 continues to operation 430 wherethe signal is constant but biased positive. At operation 432,adjustments are made to the charge compensation. At operation 426,monitoring is performed with a charge compensation slit at a turningESA. At operation 428, overall energy distribution is measured.

Referring again to FIG. 4B, from operation 428, low energy chargecompensation (CC) Jaw/Si intensity is determined at operation 436. Atoperation 438, adjustments are made for charge compensation for alow-energy CC slit signal and determined if similar to a conductingsample. If no, operation 440 returns to operation 424. If yes, operation442 leads to operation 444 to determine for stable Si intensity. Then,at operation 446, it is determined if the low-energy CC slit signal issimilar to a conducting sample. If so, the flowchart continues tooperation 420 which is a “Go” for running the analysis.

FIG. 5 illustrates a secondary ion beam path through an upperspectrometer and turning electrostatic analyzer (ESA) section, inaccordance with an embodiment of the present invention.

Referring to FIG. 5, portions of the system 100 of FIG. 1 are shown ingreater detail for a particular embodiment. The beam 110 in the slitregion 120 between ESAs 118 and 116 is split into an inner trace 110Aand an outer trace 110B. The inner trace 110A represents a conductingsample (e.g., surface potential=0V). The outer trace 110B representsidentical secondary ion distribution with +50V surface potential at thesample. The function of a secondary ion blanker 500 included in thesystem 100 is to deflect the ions into a total ion current sensor 502.

Referring again to FIG. 5, the effect of changing the sample bias from0V to +50V for identical secondary ion energy distributions isillustrated. The positive sample bias positive charging of the surface)effectively shifts the beam path toward the high-energy sensor of theenergy slit of the turning ESA system and increases the measured currenton the high energy slit jaw (see +50V sample bias outer trace). At thesame time, the low-energy slit jaw will effectively measure no current(see inner trace for 0V bias).

In an embodiment, the high energy slit sensor signal can in turn be usedto monitor the effect of a charge neutralization system (for positivesecondary ions). For negative secondary ions or electrons, a positivesurface potential results in a shift of the energy distribution towardthe low-energy slit sensor and may in the extreme effectively suppresssecondary ion emission from the sample surface altogether (as thenegative charged particles are attracted to the positive samplepotential).

FIG. 6 illustrates a secondary ion beam path for positive ions throughan upper spectrometer and turning electrostatic analyzer (ESA) section,in accordance with an embodiment of the present invention.

Referring to FIG. 6, portions of the system 100 of FIG. 1 are shown ingreater detail for a particular embodiment, with a sample location 103depicted. The trajectory trace indicates surface charging towardnegative (e.g., −20V) surface potential. That is, positive secondaryions appear to shift toward lower secondary ion energy. Secondary ionsare cut off at the low energy slit sensor.

In an embodiment, if the sample surface accumulates a negativepotential, positive secondary ions will shift toward apparent lowersecondary ion energy. This shift may occur when the overall samplepotential is changes from positive to less positive potential. The causeof such shifts may be due to one or more of the following factors: (a)an intentional sample bias used to reject low-energy secondary ions, (b)if a constant positive surface potential is induced on a semi-insulatingsample and the sample is irradiated with higher energy electrons thatcause a net negative surface charge, and/or (c) the overall sample biasis set to a more negative potential for a conducting sample to rejectlow-energy secondary ions from the measurement.

FIG. 7 illustrates an energy distribution scan toward high energy, usingthe low and high energy current sensors following turning ESA 116 asfeedback which changes the mean pass energy of the electrostaticanalyzer, in accordance with an embodiment of the present invention.

Referring to FIG. 7, portions of the system 100 of FIG. 1 are shown ingreater detail for a particular embodiment, with a sample location 103depicted. The approach involves turning ESA scan on ESA 116 only (outerESA Vo-10V, inner Vi+10V) while ESA 118 is at nominal voltages. Thesystem will not transmit the secondary ions to the mass analyzer sinceESA 118 is still tuned to the nominal pass energy.

FIGS. 8A, 8B, 8C, and 8D include a schematic illustration 800 formeasuring the secondary ion energy distribution using the electrostaticturning ESA 116, as well as the high- and low-energy slit sensorsdescribed in association with FIG. 7, in accordance with an embodimentof the present invention. In an embodiment, a same or similarexperimental setup can be used for monitoring the effectiveness of acharge compensation system as described in association with FIGS. 4A and4B. The implementation may be in conjunction with additional features tocontrol or adjust the surface potential of the sample as described inthe above approaches for adjusting the surface potential.

Referring to FIG. 8A, the energy distribution scan is on the chargecompensation (CC) slit 120 after turning ESA 116. In an exemplaryembodiment, the sample is silicon or SiGe and is well grounded. Thissetting is used for charge compensation reference as well. Energyadjustment is to be accomplished via e-beam current/energy to move ionenergy back toward the slit detector. At operation 802, chargecompensation is OFF. At operation 804, turning ESA 116 is at nominalvoltages. At operation 806, a wafer auto-z is performed and checked forOK. At a corresponding control module 850, a vision system/stageoperation 852 is used for performing operation 806. At operation 808, asilicon polarity set is loaded at standard setting values. At thecorresponding control module 850, an upper spectro (or lower spectro)operation 854 is used for performing operation 808. Al operation 810, aprimary beam energy/raster size/rotation adjustment is performed. At thecorresponding control module 850, a primary column operation 856 is usedfor performing operation 810. At operation 812, a synch with beam isperformed. At the corresponding control module 850, an upper spectrooperation 858 is used for performing operation 812. At operations 814and 816, the primary beam is blanked and then set ON, respectively. Atthe corresponding control module 850, an upper spectro operation 860 anda primary current operation 862 is used for performing operations 814and 816, respectively. At operation 818, charge compensation slit widthis set (e.g., to 3.5 mm of a 50 eV energy pass). At the correspondingcontrol module 850, a smart stage controller operation 864 is used forperforming operation 818. At operation 820, charge compensation (CC) isdetermined by measuring slit current. At the corresponding controlmodule 850, a smart stage controller operation 866 and an upper spectrocharge compensation slit current operation 868 are used for performingoperation 820. Flow from operation 820 then continues at FIG. 8B.

Referring to FIG. 8B, from operation 820, a scan block high energyoperation 822 is performed, which marks a start energy scan operation824 at ESA 116, which continues through operations 824A-824E as depictedin FIG. 8B. The charge compensation program ends operation 822 atoperation 826 with the beam blank setting at OFF. The turning ESA 116 isthen set to nominal at operation 828. Flow from operation 828 thencontinues at FIG. 8C.

Referring to FIG. 8C, from operation 828, a reverse turning ESA 116 scanoperation 830 is performed. A scan block low energy operation 832 isthen performed, which marks a start energy scan operation 834 at ESA116, which continues through operations 834A-834E as depicted in FIG.8C. The charge compensation program ends operation 832 at operation 836with the secondary beam blank setting at OFF. The turning ESA 116 isthen set to nominal at operation 838.

Referring to FIG. 8D, if further compensation is however required,further determinations may be performed at operation 840. For example,at operation 840A, if required, the charge compensation (CC) slit ismoved at constant width for a target voltage. It is to be appreciatedthat, in one embodiment, the charge compensation slit may also be usedto reject low energy Secondary ions. It is also to be appreciated that avoltage offset may be applied to enable passage of a minimum pass energyto ESA 116 and ESA 118. At operation 840B, the primary beam is stopped(OFF position).

FIG. 9 illustrates an energy distribution scan involving turning ESAscan on ESA 116 and. ESA 118 at a same voltage, in accordance with anembodiment of the present invention. Referring to FIG. 9, portions ofthe system 100 of FIG. 1 are shown in greater detail for a particularembodiment. The approach involves setting the outer portions of ESAs 116and 118 at Vo-10V. The inner portions of ESAs 116 and 118 are set atVi+10V.

Thus, embodiments include approaches to controlling the surfacepotential of a sample. In an embodiment, the potential at the samplesurface can be controlled using one or more of: (1) adjusting the biasvoltage of the sample substrate (described in greater detail below, inassociation with FIG. 10), (2) directing a beam of electrons at thesurface and varying the current delivered to the sample surface(described in greater detail below, in association with FIG. 11), (3)directing a beam of electrons at the surface and varying the impactenergy of the electrons, such that the secondary electrons coefficientis either greater than 1 (driving the sample potential more positive),or less than one (driving the sample potential more negative), (4) acombination of (2) and (3) such that the net charge flux delivered bythe electron beam can be of either polarity and variable in intensity,(5) directing a variable intensity of photons to the sample surface suchthat charge carriers are generated in the sample, varying itsconductivity (described in greater detail below, in association withFIG. 12), (6) directing a variable intensity of photons to the samplesurface such that photoelectrons are emitted from the sample, drivingthe sample potential more positive, (7) directing a variable intensityof photons away from the sample surface onto a material of highsecondary electron coefficient and flooding the sample surface withlow-energy electrons, thus driving the sample potential more negative(described in greater detail below, in association with FIGS. 13A and13B), and/or (8) the approach of (7), while applying a positive ornegative potential to the electron emitting material to control the fluxand energy of electrons striking the sample surface.

FIG. 10 is a schematic illustrating a positive potential on the sampledue to surface charging, in accordance with an embodiment of the presentinvention.

Referring to FIG. 10, a positive primary ion beam 1002 is directed at aportion 1004 of a sample 1006. Secondary ions 1008 are generated andcollected at an extraction module 1010. The sample potential of thesample 1006 (and in particular the portion 1004 of the sample 1006) canbe adjusted, e.g., by adjusting the bias voltage of the samplesubstrate.

FIG. 11 is a schematic illustrating electron beam based chargeneutralization and control, in accordance with an embodiment of thepresent invention.

Referring to FIG. 11, a positive primary ion beam 1102 is directed at aportion 1104 of a sample 1106. Secondary ions 1108 are generated andcollected at an extraction module 1110. A beam of electrons 1112 isdirected at the surface of 1104. Meanwhile, the current delivered to thesample surface is varied.

FIG. 12 is a schematic illustrating photo conductivity of the sampleusing a variable intensity photon source, in accordance with anembodiment of the present invention.

Referring to FIG. 12, a positive primary ion beam 1202 is directed at aportion 1204 of a sample 1206. Secondary ions 1208 are generated andcollected at an extraction module 1210. A variable intensity of photons1212 is directed to the surface of 1204 such that charge carriers aregenerated in the sample, varying its conductivity.

FIG. 13A is a schematic illustrating charge neutralization by use of anelectron emitting material away from the sample, in accordance with anembodiment of the present invention. Referring to FIG. 13A, a region1304A of a sample 1306A is charged positive. Electron emitters 1350A areincluded in the system. Low-energy electrons 1352A are attracted by thepositive surface potential of the region 1304A of the sample 1306A.

FIG. 13B is a schematic illustrating effective charge neutralization ifthe electron flux exceeds the primary ion induced positive surfacecharge, in accordance with an embodiment of the present invention.Referring to FIG. 13B, a region 1304B of a sample 1306B is chargeneutral. Electron emitters 1350B are included in the system. Net surfacepotential is equal to max electron energy of the electrons 1352B.Referring to FIGS. 13A and 13B collectively, a variable intensity ofphotons is directed away from the sample surface onto a material of highsecondary electron coefficient and the sample surface is flooded withlow-energy electrons, thus driving the sample potential more negative.

In a third aspect, embodiments of the present invention are directed tomethods for determining wafer backside contact resistance using multiplecapacitive height sensors.

To provide context, in metrology equipment for semiconductor wafers itis common to use either charged particle (electrons/ions) beams as aprimary exciting source, or measure the properties of charged particlesthat are emitted from the wafer, or both. Charging of the wafer cancause either errors in measuring the properties of the emitted particlesor cause the primary charged particle to be deflected into neighboringregions causing damage to these areas. To keep the wafer from chargingit is common to make contact to the backside of the wafer with aconductive electrode. However, in some instances, due to contaminationon the electrode or insulating films on the back of the wafer, there isa high resistance between the electrode and the wafer. It may bedesirable to measure the backside contact resistance in order to beassured that it is safe to apply an associated primary ion or electronbeam to the sample.

In accordance with an embodiment of the present invention, approachesfor measuring a contact resistance by using multiple capacitive heightsensors are described. To exemplify the concepts described herein, acase of a single sensor being used to measure the height to awell-grounded wafer is first considered. The sensor includes a parallelplate electrode that is within sensing distance to the wafer. Thestructure forms a parallel plate capacitor where one plate is the sensorand the other is the wafer. If an AC voltage is applied to the sensor,then current flows across the gap between the plates. The amount ofcurrent that flows across the gap is determined by the voltage, the areaof the plates, the material that separates the plates, and the distancebetween the plates.

The relevant equations to describe the current are: i_(ac)=x*C*V, wherei_(ac)=ac current (in amps), C=capacitance (in farads), V=voltage, and xis a constant that depends only on the ac frequency. Capacitance (C) inturn is defined as: C=K*E0*A/D), where C=capacitance (in farads), K=thedielectric constant of the material between the plates (e.g., Air=1.0),E0=permittivity of free space (a constant), A=area of the plates (insquare meters), D=distance between the plates (in meters). Thus, if thearea of the sensor is maintained as constant, the distance (D) isproportional to the voltage (V) divided by the current (i_(ac)). For anAC voltage, this is directly proportional to the impedance of thecircuit.

The control electronic for the sensor can be implemented several ways.For example, some capacitive sensors hold the current constant and allowthe voltage to vary. These are called constant current capacitivesensors. Other capacitive sensors hold the voltage constant and allowthe current to vary. These are constant voltage designs. There is noinherent advantage of one design over the other.

The above description assumes that the wafer is well grounded to themeasurement electronics such that the impedance that is measured is dueto the sensor to wafer gap. If the wafer is not well grounded, then thesum of the impedance due to the sensor to wafer gap and the impedance ofthe wafer to ground is what is measured. This, unfortunately, results ina distance being reported that is the sum of the wafer to sensor gap andthe error from the wafer impedance. Approaches for using a second sensoror compensating electrode to minimize the wafer to ground current havebeen described for reducing the error in the distance measurement. Thegeneral idea is to feed the second electrode with an AC voltage that isamplitude and/or phase shifted version of the drive signal that is fedto the main sensor electrode. When the net current of the two electrodesis zero then the value from the main electrode is an accurate measure ofthe gap distance.

FIG. 14 illustrates schematically some of the parameters involved inusing a compensating electrode to determine gap distance. Referring toFIG. 14, a compensating electrode 1402 is shown as coupled to a sensorsignal generator 1404. A sensor 1406 is coupled to the sensor signalgenerator 1404. The arrangement in FIG. 14 shows the relationship of thecompensating electrode 1402, the sensor signal generator 1404, and thesensor 1406 to a target 1408.

It is to be appreciated that, in accordance with an embodiment of thepresent invention, it is of interest to further determine the value ofthe signal being driven to the compensating electrode. The value is ameasure of the water impedance to ground. By determining the signal andcalibrating it with known reference impedance standards added betweenground and a conductive wafer, the signal can be calibrated such that anunknown contact resistance can be determined.

In accordance with an embodiment of the present invention, there areseveral options for performing a measurement of a compensating electrodesignal. In a first embodiment, an approach involving drive of acompensating electrode with a 180 degree shifted version of the mainelectrode signal. The amplitude is adjusted until the net current iszero (or until the distance measured by the main electrode isminimized). The amplitude of the voltage on the compensating electrodecan then be calibrated to the impedance. In a second embodiment, thedistance indicated by the main sensor is measured as the compensatingelectrode is being driven by a phase shifted version of the main signal.The distances are plotted for a number of different phase angles and acurve fitting algorithm is used to determine the phase angle for theminimum distance. The obtained phase angle is then calibrated to theimpedance. In a third embodiment, the distance of the compensatingelectrode to the wafer or the exposed area of the compensating electrodeis varied. The distance on the main electrode is measured to determinethe minimum and, thus, a value is obtained that can be calibrated to theimpedance.

Embodiments of the present invention may be provided as a computerprogram product, or software, that may include a machine-readable mediumhaving stored thereon instructions, which may be used to program acomputer system (or other electronic devices) to perform a processaccording to the present invention. A machine-readable medium includesany mechanism for storing or transmitting information in a form readableby a machine (e.g., a computer). For example, a machine-readable (e.g.,computer-readable) medium includes a machine (e.g., a computer) readablestorage medium (e.g., read only memory (“ROM”), random access memory(“RAM”), magnetic disk storage media, optical storage media, flashmemory devices, etc.), a machine (e.g., computer) readable transmissionmedium (electrical, optical, acoustical or other form of propagatedsignals (e.g., infrared signals, digital signals, etc.)), etc.

FIG. 15 illustrates a diagrammatic representation of a machine in theexemplary form of a computer system 1500 within which a set ofinstructions, for causing the machine to perform any one or more of themethodologies discussed herein, may be executed. In alternativeembodiments, the machine may be connected (e.g., networked) to othermachines in a Local Area Network (LAN), an intranet, an extranet, or theInternet. The machine may operate in the capacity of a server or aclient machine in a client-server network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a server, a network router, switch or bridge, or any machinecapable of executing a set of instructions (sequential or otherwise)that specify actions to be taken by that machine. Further, while only asingle machine is illustrated, the term “machine” shall also be taken toinclude any collection of machines (e.g., computers) that individuallyor jointly execute a set (or multiple sets) of instructions to performany one or more of the methodologies discussed herein. For example, inan embodiment, a machine is configured to execute one or more sets ofinstruction for measuring a sample by secondary ion mass spectrometry(SIMS). In an embodiment, the computer system 1500 is suitable for usewith a system such as the SIMS system depicted and described inassociation with FIG. 1.

The exemplary computer system 1500 includes a processor 1502, a mainmemory 1504 (e.g., read-only memory (ROM), flash memory, dynamic randomaccess memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM(RDRAM), etc.), a static memory 1506 (e.g., flash memory, static randomaccess memory (SRAM), etc.), and a secondary memory 1518 (e.g., a datastorage device), which communicate with each other via a bus 1530.

Processor 1502 represents one or more general-purpose processing devicessuch as a microprocessor, central processing unit, or the like. Moreparticularly, the processor 1502 may be a complex instruction setcomputing (CISC) microprocessor, reduced instruction set computing(RISC) microprocessor, very long instruction word (VLIW) microprocessor,processor implementing other instruction sets, or processorsimplementing a combination of instruction sets. Processor 1502 may alsobe one or more special-purpose processing devices such as an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA), a digital signal processor (DSP), network processor, or thelike. Processor 1502 is configured to execute the processing logic 1526for performing the operations discussed herein.

The computer system 1500 may further include a network interface device1508. The computer system 1500 also may include a video display unit1510 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)),an alphanumeric input device 1512 (e.g., a keyboard), a cursor controldevice 1514 (e.g., a mouse), and a signal generation device 1516 (e.g.,a speaker).

The secondary memory 1518 may include a machine-accessible storagemedium (or more specifically a computer-readable storage medium) 1531 onwhich is stored one or more sets of instructions (e.g., software 1522)embodying any one or more of the methodologies or functions describedherein. The software 1522 may also reside, completely or at leastpartially, within the main memory 1504 and/or within the processor 1502during execution thereof by the computer system 1500, the main memory1504 and the processor 1502 also constituting machine-readable storagemedia. The software 1522 may further be transmitted or received over anetwork 1520 via the network interface device 1508.

While the machine-accessible storage medium 1531 is shown in anexemplary embodiment to be a single medium, the term “machine-readablestorage medium” should be taken to include a single medium or multiplemedia (e.g., a centralized or distributed database, and/or associatedcaches and servers) that store the one or more sets of instructions. Theterm “machine-readable storage medium” shall also be taken to includeany medium that is capable of storing or encoding a set of instructionsfor execution by the machine and that cause the machine to perform anyone or more of the methodologies of the present invention. The term“machine-readable storage medium” shall accordingly be taken to include,but not be limited to, solid-state memories, and optical and magneticmedia.

Thus, systems and approaches for semiconductor metrology and surfaceanalysis using Secondary Ion Mass Spectrometry (SIMS) have beendescribed.

In an embodiment, a secondary ion mass spectrometry (SIMS) systemincludes a sample stage. A primary ion beam is directed to the samplestage. An extraction lens is directed at the sample stage. Theextraction lens is configured to provide a low extraction field forsecondary ions emitted from a sample on the sample stage. A magneticsector spectrograph is coupled to the extraction lens along an opticalpath of the SIMS system. The magnetic sector spectrograph includes anelectrostatic analyzer (ESA) coupled to a magnetic sector analyzer(MSA).

In one embodiment, the sample stage includes a Faraday cup.

In one embodiment, the SIMS system further includes a plurality ofdetectors spaced along a plane of the magnetic sector spectrograph.

In one embodiment, the plurality of detectors is for detecting acorresponding plurality of different species from a beam of thesecondary ions emitted from the sample.

In one embodiment, the SIMS system further includes a first additionalESA coupled along the optical path of the SIMS system, between theextraction lens and the ESA of the magnetic sector spectrograph, and asecond additional ESA coupled along the optical path of the SIMS system,between the first additional ESA and the ESA of the magnetic sectorspectrograph.

In one embodiment, the first additional ESA is configured to spread abeam of the secondary ions emitted from the sample, and wherein thesecond additional ESA is configured to concentrate the beam of thesecondary ions received from the first additional ESA.

In one embodiment, the SIMS system further includes one or more slitsalong the optical path of the SIMS system, between the first additionalESA and the second additional ESA.

In one embodiment, the first additional ESA, the second additional ESA,and the one or more slits are included in a charge compensation systemof the SIMS system.

In one embodiment, the first additional ESA and the second additionalESA together direct the optical path of the SIMS system from above thesample stage to below the sample.

In one embodiment, a magnet of the MSA is located below the samplestage.

In one embodiment, the low extraction field has an absolute value of atmost approximately 10 Volts/mm.

In one embodiment, the SIMS system further includes a transfer robotcoupled to the sample stage.

In one embodiment, the SIMS system further includes a receptacle coupledto the transfer robot, the receptacle comprises calibration standards.

In an embodiment, a method of measurement and control of the surfacepotential of a sample involves measuring kinetic energy of chargedparticles emitted from a surface of a sample. The method also involvesdetermining a shift in kinetic energy of the charged particles. Thesurface potential of the surface of the sample is changed in responsethe shift in kinetic energy of the charged particles.

In one embodiment, the surface potential of the surface of the samplecomprises adjusting a bias voltage of an electrode supporting thesample.

In one embodiment, changing the surface potential of the surface of thesample comprises directing a beam of electrons at the surface of thesample.

In one embodiment, directing the beam of electrons at the surface of thesample comprises varying a current delivered to the surface of thesample.

In one embodiment, directing the beam of electrons at the surface of thesample comprises varying an impact energy of the beam of electrons atthe surface of the sample.

In one embodiment, changing the surface potential of the surface of thesample comprises directing photons of variable intensity to the samplesurface.

In one embodiment, directing photons of variable intensity to the samplesurface comprises generating charge carriers in the sample to vary aconductivity of the sample.

In one embodiment, directing photons of variable intensity to the samplesurface comprises emitting photoelectrons from the sample to drive asample potential to more positive.

In one embodiment, changing the surface potential of the surface of thesample comprises directing photons of variable intensity away from thesample surface.

In one embodiment, the method further includes, subsequent to changingthe surface potential of the surface of the sample, performing asecondary ion mass spectrometry (SIMS) measurement of the surface of thesample.

In an embodiment, a method of determining wafer backside contactresistance involves measuring a gap distance value of a surface of awafer based on a comparison of a main capacitive sensor electrode drivenwith a first drive signal and a compensating capacitive sensor electrodedriven with a second drive signal that is amplitude or phase shifted ascompared to the first drive signal. A value of the second drive signalis measured. The value of the second drive signal is calibrated to areference impendence standard to determine an impedance value of thewafer to ground. A contact resistance value is determined for thesurface of the wafer based on the gap distance value and the impedancevalue of the wafer to ground.

In one embodiment, measuring the value of the second drive signalcomprises driving the compensating capacitive sensor with the seconddrive signal that is a 180 degree shifted version of the first drivesignal, and adjusting an amplitude of the second drive signal to obtainan amplitude value when a net current of the main capacitive sensorelectrode and the compensating capacitive sensor electrode is zero, andwherein calibrating the value of the second drive signal to thereference impendence standard comprises calibrating the amplitude valueto the reference impendence standard.

In one embodiment, measuring the value of the second drive signalcomprises driving the compensating capacitive sensor with the seconddrive signal that is a phase shifted version of the first drive signal,and adjusting a phase angle of the second drive signal to obtain a phaseangle value when a minimum gap distance value is obtained, and whereincalibrating the value of the second drive signal to the referenceimpendence standard comprises calibrating the phase angle value to thereference impendence standard.

In one embodiment, measuring the value of the second drive signalcomprises varying a distance of the compensating capacitive sensorelectrode from the surface of the wafer to obtain minimum gap distancevalue, and wherein calibrating the value of the second drive signal tothe reference impendence standard comprises calibrating the minimum gapdistance value to the reference impendence standard.

In one embodiment, measuring the gap distance value comprises measuringthe gap distance value when a net current of the main capacitive sensorelectrode and the compensating capacitive sensor electrode is zero.

In one embodiment, the method further includes contacting a conductiveelectrode to the surface of the wafer, and directing a charged particlebeam to a second surface of the wafer when the contact resistance valuefor the surface of the wafer is below a threshold value.

In one embodiment, directing the charged particle beam to the secondsurface of the wafer comprises initiating a secondary ion massspectrometry (SIMS) measurement of the second surface of the wafer.

1. A secondary ion mass spectrometry (SIMS) system, comprising: a samplestage; a primary ion source and ion optics for producing and directing aprimary ion beam to the sample stage; an extraction lens directed at thesample stage, the extraction lens configured to provide a low extractionfield for secondary ions emitted from a sample on the sample stage; anda magnetic sector spectrograph coupled to the extraction lens along anoptical path of the SIMS system, the magnetic sector spectrographcomprising an electrostatic analyzer (ESA) coupled to a magnetic sectoranalyzer (MSA).
 2. The SIMS system of claim 1, wherein the sample stagecontains a Faraday cup.
 3. The SIMS system of claim 1, furthercomprising: a plurality of detectors spaced along a plane of themagnetic sector spectrograph.
 4. The SIMS system of claim 3, wherein theplurality of detectors is for detecting a corresponding plurality ofdifferent species from a beam of the secondary ions emitted from thesample.
 5. The SIMS system of claim 1, further comprising: a firstadditional ESA coupled along the optical path of the SIMS system,between the extraction lens and the ESA of the magnetic sectorspectrograph; and a second additional ESA coupled along the optical pathof the SIMS system, between the first additional ESA and the ESA of themagnetic sector spectrograph.
 6. The SIMS system of claim 5, wherein thefirst additional ESA is configured to spread a beam of the secondaryions emitted from the sample, and wherein the second additional ESA isconfigured to concentrate the beam of the secondary ions received fromthe first additional ESA.
 7. The SIMS system of claim 6, furthercomprising: one or more slits along the optical path of the SIMS system,between the first additional ESA and the second additional ESA.
 8. TheSIMS system of claim 7, wherein the first additional ESA, the secondadditional ESA, and the one or more slits are included in a chargecompensation system of the SIMS system.
 9. The SIMS system of claim 5,wherein the first additional ESA and the second additional ESA togetherdirect the optical path of the SIMS system from above the sample stageto below the sample.
 10. The SIMS system of claim 9, wherein a magnet ofthe MSA is located below the sample stage.
 11. The SIMS system of claim1, wherein the low extraction field has an absolute value of at mostapproximately 10 Volts/mm.
 12. The SIMS system of claim 1, furthercomprising: a transfer robot coupled to the sample stage.
 13. The SIMSsystem of claim 12, further comprising: a receptacle coupled to thetransfer robot, the receptacle being capable of containing calibrationor reference wafers or wafer-like carriers capable of containingcalibration or reference samples. 14.-30. (canceled)